Method for producing hardened structural elements

ABSTRACT

The invention relates to a method for producing a hardened steel component with a coating composed of zinc or a zinc alloy; a blank is stamped out of a sheet coated with the zinc or zinc alloy, the stamped-out blank is heated to a temperature ≧Ac3 and if need be, kept at this temperature for a predetermined time in order to induce the formation of austenite, and then the heated blank is transferred to a forming die, is formed in the forming die, and is cooled at a speed that is greater than the critical hardening speed and thus hardened; the steel material is adjusted in a transformation-delaying fashion so that a quench hardening through transformation of austenite into martensite takes place at a forming temperature that lies in the range from 450° C. to 700° C.; after the heating and before the forming, an active cooling takes place at &gt;15K/s.

FIELD OF THE INVENTION

The invention relates to a method for producing hardened, corrosion-protected components.

BACKGROUND OF THE INVENTION

It is known that particularly in automobiles, so-called press-hardened components composed of sheet steel are used. These press-hardened components composed of sheet steel are high-strength components that are particularly used as safety components in the region of the vehicle body. In this connection, the use of these high-strength steel components makes it possible to reduce the material thickness relative to a normal-strength steel and thus to achieve low vehicle body weights.

In press-hardening, there are basically two different possibilities for manufacturing such components. They are divided into the so-called direct and indirect methods.

In the direct method, a sheet steel blank is heated to a temperature greater than the so-called austenitization temperature and if need be, kept at this temperature until a desired degree of austenitization is achieved. Then, this heated blank is transferred to a forming die and in this forming die, is shaped into the finished component in a one-step forming process and in so doing, by means of the cooled forming die, simultaneously cooled at a speed that is greater than the critical hardening speed. This produces the hardened component.

In the indirect method, first, possibly in a multi-step forming process, the component is formed until it is almost completely finished. This formed component is then likewise heated to a temperature greater than the austenitization temperature and if need be, kept at this temperature for a desired, necessary period of time.

Then this heated component is transferred and inserted into a forming die that already has the dimensions of the component or the final dimensions of the component, if need be taking into account the thermal expansion of the preformed component. After the closing of the in particular cooled die, the preformed component is consequently cooled in this die at a speed that is greater than the critical hardening speed and is thus hardened.

In this connection, the direct method is somewhat simpler to implement, but only permits shapes that can actually be produced by means of a one-step forming process, i.e. relatively simple profile shapes.

The indirect process is somewhat more complex, but is also able to produce more complex shapes.

In addition to the need for press-hardened components, a need has also arisen to produce such components not out of uncoated sheet steel, but rather to provide such components with a corrosion protection layer.

In the automotive field, the corrosion protection layer can be composed either of rather infrequently used aluminum or aluminum alloys or of significantly more frequently used zinc-based coatings. In this connection, zinc has the advantage that it provides not just a barrier protection layer like aluminum does, but also a cathodic corrosion protection. In addition, zinc-coated press-hardened components fit better into the overall corrosion protection concept of vehicle bodies since in the construction technique that is currently popular, they are generally galvanized as a whole. In this respect, it is possible to reduce or eliminate contact corrosion.

But both methods could involve disadvantages that have also been discussed in the prior art. In the direct method, i.e. the hot forming of press-hardened steels with zinc coatings, microcracks (10 μm to 100 μm) or even macrocracks occur in the material; the microcracks occur in the coating and the macrocracks even extend through the entire cross-section of the sheet. Components of this kind with macrocracks are unsuitable for further use.

In the indirect process, i.e. cold forming with a subsequent hardening and remaining forming, microcracks in the coating can also occur, which are also undesirable, but far less pronounced.

Thus far—except for one component produced in Asia—zinc-coated steels have not been used in the direct method, i.e. hot forming. With this method, preference is given to using steels with an aluminum/silicon coating.

An overview is given in the publication “Corrosion resistance of different metallic coatings on press hardened steels for automotive”, Arcelor Mittal Maiziere Automotive Product Research Center F-57283 Maiziere-Les-Mez. This publication states that for the hot forming process, there is an aluminized boron/manganese steel that is sold commercially under the name Usibor 1500P. In addition, steels that are pre-coated with zinc for purposes of cathodic corrosion protection are sold for the hot forming method, namely galvanized Usibor GI, which has a zinc coating containing small percentages of aluminum, and a so-called galvannealed, coated Usibor GA, which has a zinc coating containing 10% iron.

It is also noted that the zinc/iron phase diagram shows that above 782° C., there is a larger region in which liquid zinc-iron phases occur as long as the iron content is low, in particular less than 60%. But this is also the temperature range in which the austenitized steel is hot formed. It is also noted that if the forming occurs at a temperature greater than 782° C., then there is a high risk of stress corrosion due to liquid zinc, which presumably penetrates into the grain boundaries of the base steel, resulting in macrocracks in the base steel. Furthermore, at iron contents of less than 30% in the coating, the maximum temperature for the forming of a safe product without macrocracks is less than 782° C. This is the reason why direct forming methods are not used with these steels, but instead the indirect forming method is used. This is intended to bypass the above-mentioned problem.

Another possibility for bypassing this problem should lie in using galvannealed, coated steel, which is because the iron content of 10% that was already present at the beginning and the absence of a Fe₂Al₅ bather layer lead to a more homogeneous formation of the coating out of predominantly iron-rich phases. This results in a reduction or elimination of zinc-rich, liquid phases.

“‘STUDY OF CRACKS PROPAGATION INSIDE THE STEEL ON PRESS HARDENED STEEL ZINC BASED COATINGS’, Pascal Drillet, Raisa Grigorieva, Gregory Leuillier, Thomas Vietoris, 8th International Conference on Zinc and Zinc Alloy Coated Steel Sheet, GALVATECH 2011—Conference Proceedings, Genova (Italy), 2011’ indicates that galvanized sheets cannot be processed in the direct method.

EP 1 439 240 B1 has disclosed a method for hot forming a coated steel product; the steel material has a zinc or zinc alloy coating on the surface of the steel material and the steel base material with the coating is heated to a temperature of 700° C. to 1000° C. and hot formed; before the steel base material with the zinc or zinc alloy coating is heated, the coating has an oxide layer that is chiefly composed of zinc oxide in order to prevent the zinc from vaporizing during the heating. A special process sequence is provided for this purpose.

EP 1 642 991 B1 has disclosed a method for hot forming a steel in which a component composed of a boron/manganese steel is heated to a temperature at the Ac₃ point or higher, is kept at this temperature, and then the heated steel sheet is formed into the finished component; the formed component is quenched through cooling from the forming temperature during the forming or after the forming in such a way that the cooling rate at the MS point at least corresponds to the critical cooling rate and the average cooling rate of the formed component from the MS point to 200° C. lies in the range from 25° C./s to 150° C./s.

The applicant's patent EP 1 651 789 B1 has disclosed a method for manufacturing hardened components out of sheet steel; according to this method, formed parts composed of a sheet steel that is provided with a cathodic corrosion-protection layer are cold formed and undergo a heat treatment for purposes of austenitization; before, during, or after the cold forming of the formed part, a final trimming of the formed part and required punching procedures or production of a hole pattern are carried out and the cold forming as well as the trimming and punching and arrangement of the hole pattern on the component are carried out 0.5% to 2% smaller than the dimensions that the final hardened component should have; the formed part, which has been cold formed for the heat treatment, is then heated in contact with atmospheric oxygen in at least some regions to a temperature that permits an austenitization of the steel material and the heated component is then transferred to a die and in this die, a so-called form hardening is carried out in which the contacting and pressing (holding) of the component by the form hardening dies cause the component to be cooled and thus hardened and the cathodic corrosion protection coating is composed of a mixture of essentially zinc and additionally, one or more oxygen-affine elements. As a result, on the surface of the corrosion protection coating, an oxide skin composed of the oxygen-affine elements forms during the heating, which protects the cathodic corrosion protection layer, in particular the zinc layer. In addition, in the method, the scale reduction of the component with regard to its final geometry takes into account the thermal expansion of the component so that neither a calibration nor a forming are required during the form hardening.

The applicant's patent WO 2010/109012 A1 has disclosed a method for manufacturing partially hardened steel components in which a blank composed of a hardenable steel sheet is subjected to a temperature increase that is sufficient for a quench hardening and after a desired temperature is reached and if need be, after a desired holding time, the blank is transferred to a forming die in which the blank is formed into a component and quench hardened at the same time or the blank is cold formed and the component resulting from the cold forming is then subjected to a temperature increase, with the temperature increase being carried out so that a component temperature is reached that is required for a quench hardening and the component is then transferred to a die in which the heated component is cooled and thus quench hardened; during the heating of the blank or component for the purpose of increasing the temperature to a temperature required for the hardening, in the regions that should have a lower hardness and/or a higher ductility, absorption masses are placed or are spaced apart from these regions by a narrow gap; the absorption masses, with regard to their expansion and thickness, their thermal conductivity, and their thermal capacity and/or with regard to their emissivity, are especially dimensioned so that the thermal energy acting on the component in the regions of the component that remain ductile flows through the component into the absorption mass so that these regions remain cooler and in particular, the temperature required for hardening is not reached or is only partially reached so that these regions cannot harden or can harden only partially.

DE 10 2005 003 551 A1 has disclosed a method for hot forming and hardening a steel sheet in which a steel sheet is heated to a temperature above the Ac₃ point, then undergoes a cooling to a temperature in the range from 400° C. to 600° C., and is only formed after reaching this temperature range. This reference, however, does not mention the crack problem or a coating and also does not describe a martensite formation. The object of the invention therein is the formation of intermediary structures, so-called bainite.

The object of the invention is to create a method for manufacturing sheet steel components with a corrosion protection layer in which the crack formation is reduced or eliminated and a sufficient corrosion protection is nevertheless achieved.

SUMMARY OF THE INVENTION

The above-described effect of crack formation due to liquid zinc, which penetrates the steel in the region of the grain boundaries, is also known as so-called “liquid metal embrittlement” or “liquid metal assisted cracking.”

By contrast with the course taken in the prior art of using the indirect method even with simple geometries due to “liquid metal embrittlement,” the invention takes a more advantageous course by using the direct method in which a blank coated with zinc or a zinc alloy is heated, is formed after the heating, and is quench hardened.

According to the discovery on which the invention is based, as little molten zinc as possible must come into contact with austenite during the forming phase, i.e. the introduction of stress. According to the invention, therefore, the forming must be carried out below the peritectic temperature of the iron/zinc system (melt, ferrite, gamma phase). In order to still be able to ensure a quench hardening in this case, the composition of the steel alloy as part of the conventional composition of a manganese/boron steel (22 MnB5) is adjusted so that a quench hardening is carried out and in so doing, by means of a delayed transformation of the austenite into martensite, the presence of austenite is achieved even at the lower temperature below 780° C. or lower so that at the moment in which mechanical stress is introduced into the steel by the forming, which in connection with austenite and molten zinc would lead to “liquid metal embrittlement,” no liquid zinc phases or very little of them are present. Therefore, by means of a boron/manganese steel that is adjusted in accordance with the alloy elements, it succeeds in achieving a sufficient quench hardening without provoking an excessive or damaging crack formation.

In particular, the cooling can take place with air jets; the blowing of the air jets can be controlled by means of pyrometers, which are provided, for example, outside the press and the furnace in a separate piece of equipment in the same way as the corresponding jets.

The cooling possibilities in this case are not limited to air jets; it is also possible to use cooled tables on which the blanks are correspondingly positioned so that the blanks come to lie on cooled regions of the table and are brought into thermally conductive contact, for example, by means of pressure or suction.

It is also conceivable to use a cooling press in which the flat blanks conceivably permit the press geometry to be simple and favorable; the regions of the die in which the blank is to be cooled are correspondingly liquid-cooled. Blanks that are heated all over can consequently be cooled all over in corresponding devices; the all-over cooling can be provided by means of the above-described tables and by means of the above-described intermediate presses and also by means of simple spraying, blowing, or immersion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained below in conjunction with the drawings.

FIG. 1: shows the time/temperature curve in the cooling between the furnace and the forming procedure;

FIG. 2: shows the zinc/iron diagram;

FIG. 3: shows depictions of ground cross-sections of the surface of specimens, with and without intermediate cooling;

FIG. 4: is a time temperature transformation diagram with a simplified depiction of the cooling curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, a conventional boron/manganese steel (e.g. 22MnB5) for use as a press-hardened steel material is adjusted with regard to the transformation of the austenite into other phases so that the transformation moves into deeper regions and martensite can be produced.

Steels of the following alloy composition are therefore suitable for the invention (all data in mass %):

C [%] Si [%] Mn [%] P [%] S [%] Al [%] Cr [%] Ti [%] B [%] N [%] 0.22 0.19 1.22 0.0066 0.001 0.053 0.26 0.031 0.0025 0.0042 the rest being made up of iron and inevitable smelting-related impurities

In steels of this kind, in particular the alloy elements boron, manganese, carbon, and optionally chromium and molybdenum are used as transformation inhibitors.

Steels of the following general alloy composition are also suitable for the invention (all data in mass %):

Carbon (C) 0.08-0.6  Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.5  Chromium (Cr) 0.02-0.6  Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02  Phosphorus (P) <0.01 Sulfur(S) <0.01 Molybdenum (Mo) <1   the rest being made up of iron and inevitable smelting-related impurities

Steels of the following composition have turned out to be particularly suitable (all data in mass %):

Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3  Titanium (Ti) 0.03-0.04 Nitrogen (N)  <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1   the rest being made up of iron and inevitable smelting-related impurities

The alloy elements functioning as transformation inhibitors are adjusted to reliably achieve a quench hardening, i.e. a rapid cooling with a cooling speed that is greater than the critical hardening speed even below 780° C. This means that in this case, work is carried out below the peritectic point of the zinc/iron system, i.e. mechanical stress is exerted only below the peritectic point. This also means that at the moment in which mechanical stress is exerted, liquid zinc phases that could come into contact with the austenite are no longer present.

In addition, after the heating of the blank, a holding phase in the temperature range of the peritectic point can be provided according to the invention so that the solidification of the zinc coating is promoted and advanced before the subsequent forming procedure is carried out.

FIG. 1 shows a favorable temperature curve for an austenitized steel sheet; it is clear that after the heating to a temperature greater than the austenitization temperature, the passage of a corresponding amount of time in a cooling device already achieves a certain amount of cooling. This is followed by a rapid intermediate cooling step. The intermediate cooling step is advantageously carried out with cooling speeds of at least 15 K/s, preferably at least 30 K/s, even more preferably at least 50 K/s. Then the blank is transferred to the press and the forming and hardening are carried out.

FIG. 3 shows the difference in the crack formation. Without intermediate cooling, cracks form that extend into the steel material; with the intermediate cooling, only surface cracks in the coating occur; these are not critical, however.

With the invention, it is therefore possible to reliably achieve an inexpensive hot forming method for steel sheets coated with zinc or zinc alloys, which on the one hand, induces a quench hardening and on the other hand, reduces or eliminates microcrack and macrocrack formation that leads to component damage. 

1. A method for producing a hardened steel component, comprising: stamping a blank out of a sheet of steel material coated with zinc or a zinc alloy; heating the stamped-out blank to a temperature ≧Ac3 and if need be, keeping the stamped-out blank at this temperature for a predetermined time in order to induce the formation of austenite; then transferring the heated blank to a forming die; forming the blank in the forming die; cooling the blank in the forming die at a speed that is greater than a critical hardening speed and thus hardening the formed blank; adjusting the steel material in a transformation-delaying fashion so that a quench hardening through transformation of austenite into martensite takes place at a forming temperature that lies in a range from 450° C. to 700° C.; and after the heating and before the forming, an active cooling takes place by cooling the blank or parts of the blank at a cooling speed >15K/s.
 2. The method according to claim 1, wherein the steel material comprises the elements boron, manganese, carbon, and optionally chromium and molybdenum as transformation inhibitors.
 3. The method according to claim 1, comprising using a steel material of the following composition (all data in mass %): Carbon (C) 0.08-0.6  Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.5  Chromium (Cr) 0.02-0.6  Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02  Phosphorus (P) <0.01 Sulfur(S) <0.01 Molybdenum (Mo) <1

the rest being made up of iron and inevitable smelting-related impurities.
 4. The method according to claim 1, comprising using a steel material of the following composition (all data in mass %): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3  Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1

the rest being made up of iron and inevitable smelting-related impurities.
 5. The method according to claim 1, comprising heating the blank in a furnace to a temperature >Ac3 and keeping the blank at this temperature for a predetermined time and then cooling the blank to a temperature between 500° C. and 600° C. in order to achieve a solidification of the zinc layer and then transferring the blank into the forming die and forming the component therein.
 6. The method according to claim 1, comprising carrying out the active cooling so that the cooling rate is >30 K/s.
 7. The method according to claim 6, comprising carrying out the active cooling so that the cooling takes place at more than 50 K/s.
 8. The method according to claim 1, comprising producing the active cooling by blowing with air or gas, spraying with water or other cooling liquids, immersion in water or other cooling liquids, or by placing cooler solid components against the blank.
 9. The method according to claim 1, comprising monitoring the cooling progress and/or the insertion temperature into the forming die using pyrometers, and correspondingly controlling the cooling. 